350 STRUCTURE OF THE VERTEBRATES 



and the articular, both cartilage bones. In at least one known 

 form, a second point of articulation has developed between the 

 dentary and the squamosal. Both are functional, thus disposing 

 of the old argument that the transition animal "could neither 

 eat nor hear". 



Until comparatively recent research showed the embryologi- 

 cal mechanism for the evolution of the vertebrae, this was an- 

 other point which depended entirely upon palaeontology for 

 its proof. It has been stated that in the primitive condition there 

 are two vertebrae in each body segment (an inter centrum and 

 a neurocentrum) with the rib developing in the myoseptum. 

 This condition is found in the earliest amphibia. The intercen- 

 trum grows progressively smaller, until in the mammal it is 

 present only as small chevron bones in the tail region. In tail- 

 less animals they are entirely lacking. 



Embryologically the vertebrae begin as eight centers of carti- 

 lage in each segment, forming two incomplete rings and cor- 

 responding to the isolated condition of the vertebrae of the 

 cyclostomes. Soon two rings are formed in the notochordal 

 sheaths, the anterior ring of each segment developing a neural 

 arch. These neurocentra push anteriorly, crowding the soft tissues 

 of the intercentra downward, until each neurocentrum articulates 

 with the next anterior one. In the Therapsida the intercentra 

 are left as small triangular bones between the neurocentra. In 

 the mammals they are pushed out completely except when left 

 as chevron bones. 



Detailed research has deciphered the history of each bone of 

 the body in its evolution from fish to mammal; the changes 

 and the homologies of the muscles are known; and even the 

 minutiae of embryology have been studied and found to fit per- 

 fectly into the picture. At best these facts can be but briefly 

 stated in an elementary text, and the student is referred to 

 more technical books and papers. 



F. INIammals 



Fossils have been found in the Triassic Period which appear 

 to be mammals or an intermediate stage. They are probably the 

 forerunners of the mammals which are known from the Juras- 



